Figure 4: a) Illustration of the nutshell structure and movement of water droplet inside. b) Comparison of water transfer time from outer shell surface to inner shell surface of four NSs (thickness 1.6 mm); c) Evaluation of Water absorption and desorption kinetics of AS, FS, PS, WS at varying time intervals. d) Sketch of water transportation along cross-sectional and surface regions of WS, emphasizing higher mobility in surface-driven transport. g) Illustration of outer WS with a dense structure and inner WS with enhanced porosity, making convergent-divergent channels.
To evaluate the ability of the NSs to absorb and release water, each of the NS was immersed in DI water independently for a couple of days, and the resulting weight was periodically measured. Subsequently, the wet NS samples were subject to the ambient atmosphere (T= 25 ºC and 30% Relative humidity) to evaluate the desorption of water because of the spontaneous evaporation. Multiple samples (n=8) were examined to validate the result. Figure 4 (c) demonstrates the capacity to absorb and release DI water over different durations. As anticipated, AS has the greatest capacity for absorption because of its exceptionally porous structure and lowest desorption rate due to its abundant vascular bundle,[45] causing it to retain water for a longer duration. In contrast, WS desorbs the water quicker than other NS. This might be attributed to the higher concentration of hydrophobic lignin contents in WS, as presented in Table-S 1 . The lignin and cellulose contain hydrophilic hydroxyl (-OH), carboxyl (-COOH) and carbonyl (C=O) functional groups and the aromatic hydrophobic functional groups.[46] It has been proven that the balanced combination of hydrophilic and hydrophobic framework leads to enhanced evaporation rate.[47,48] The cross-section of the NSs was also experimented with for the water flow. Except for AS, all other NS exhibits reduced water mobility along the dense cross-section compared to the surface, as schematically illustrated in Figure 4 (d). The outer shell surface seems denser than the inner shell surface, as schematically illustrated in Figure 4 (e). Therefore, the time duration of water transport differs between the exterior and interior surfaces.
When each of the nutshells (NSs) is immersed in a small 1 ml droplet of deionized (DI) water, the bottom surface becomes saturated, inducing capillary flow to initiate and gradually spread the DI water to the top surface. This process can be accelerated by placing another tiny 0.3 ml DI water droplet on its upper surface. Capillary actions drive the bottom water to transfer upwards, while gravity helps the top water flow downwards. The phenomenon enabled by capillary forces and the directed gravitational flow of water via micro/nanochannels is attributed to the flow of water without using pumps. Subsequently, the whole porous structure transforms from a dry to a completely wet state, and water starts to evaporate naturally from the wet NS surfaces, as illustrated in Figure 5 (a) . Under identical experimental conditions, open circuit voltage (Voc) and short circuit current density (Jsc) are recorded after the NSs are stabilized for 40 minutes at ambient conditions with 25% relative humidity (RH) and 25°C, for the G-NS-G structure containing AS, FS, PS, WS as illustrated in